The dominant form of burner used in residential and commercial hot water heaters and boilers is the “can-style” premix burner. Can-style premix burners typically are composed of perforated, rolled alloy or metal fiber formed into a “can” shape. The perforations, which have a fixed-geometry, serve as burner ports. The burners are provided with a mixture of fuel gas and combustion air (sometimes referred to herein as a “premix”). The premix includes all of the combustion air and all of the fuel to be combusted in the burner. The premix passes through the ports to a flame zone outside the can where the premix is combusted.
In order to ensure that the fuel is substantially completely burned in the burner and fired chamber, and that unburned fuel is not emitted to the atmosphere, the premix typically includes a sufficient quantity of combustion air to produce an exhaust gas flue reading of about 2-3% excess oxygen after the fuel is burned. This excess oxygen typically is provided as a constituent of an equivalent percentage of excess combustion air. As such, the premix typically includes about 10-15% excess combustion air. In operation, the burners burn the premix and emit water vapor, carbon dioxide, nitrogen, excess combustion oxygen and heat. The foregoing emissions (sometimes referred to herein as “flue gases”) are vented through a flue to the atmosphere. A portion of the heat is used to heat the water in the boiler and the rest of the heat is vented to the atmosphere via the flue gases. Other forms of premix burner, for ceramic plate burners, operate in substantially the same way.
One challenge facing the burner industry is to provide a premix burner that produces both low oxides of nitrogen (“NOx”) and high thermal efficiency. NOx production increases exponentially with increasing flame temperature. As such, reducing flame temperature can significantly lower NOx production. Known techniques for reducing flame temperature, however, can have an undesirable effect on thermal efficiency. One such technique involves simply providing additional excess combustion air to the premix, so that the premix contains, for example, about 30-40% excess combustion air. The additional excess combustion air provides an additional thermal mass that quenches the burner flame (that is, absorbs heat from the flame) when the fuel is burned, thereby reducing the flame temperature and, consequently, reducing NOx emissions. The heated, excess combustion air is vented to the atmosphere as a flue gas. Although this technique has been effective in reducing NOx emissions, it compromises the burner's thermal efficiency because a substantial amount of heat that otherwise could have been used to heat the water in the boiler is instead transferred to the excess combustion air and lost when the excess combustion air is vented to the atmosphere.
The present disclosure illustrates and describes an exemplary system for controlling NOx production in a premix burner by recycling a portion of the flue gases into the combustion air. The system includes a flue gas recirculation line having a flow restrictor therein, a fresh air line having a flow restrictor therein and means for drawing recycled flue gas and fresh air through the flue gas recirculation line and fresh air line, respectively, in a predetermined ratio.
In the illustrated embodiment, an air/fuel mixing apparatus (sometimes referred to herein as a “premix engine”) provides a premix to a premix burner where the premix is combusted. A flue associated with the burner carries flue gases away from the burner. A flue gas recirculation line is in fluid communication with flue and the premix engine so that the premix engine may draw in a portion of the flue gas as a component of the combustion air. More particularly, the flue gas recirculation line is connected at one end to the flue and at the other end to an inlet of the premix engine or an intervening combustion air line. Similarly, a fresh air line is in fluid communication with the atmosphere or another source of fresh air and the premix engine so that the premix engine may draw in fresh air as component of the combustion air. More particularly, the fresh air line is open at one end to the atmosphere or other source of fresh air and connected at the other end to an inlet of the premix engine or an intervening combustion air line. A flue gas flow restrictor is installed in the flue gas recirculation line between the flue and the premix engine or intervening combustion air line. Similarly, a fresh air flow restrictor is installed in the fresh air line between the fresh air supply and the premix engine or intervening combustion air line. The flow restrictors have fixed flow geometry, and they are sized so that the premix engine can draw recycled flue gas from the flue gas recirculation line and fresh air from the fresh line in a predetermined proportion.
Additional features of and modifications to the present disclosure will become apparent to those skilled in the art upon consideration of the following detailed description of illustrative embodiments exemplifying the best mode of implementing the disclosed system as presently perceived.